Radio base stations are used in mobile communications technology to establish radio communication links between a mobile station, such as a mobile phone or the like, and a communications network in order to transfer communications data into telephony or other communications networks and vice versa.
A radio base station comprises two basic building blocks, the so-called radio equipment controller (REC) and the radio equipment (RE) itself. This decomposition provides deployment flexibility for the mobile network operators. The decomposition requires the definition of the interfaces between the radio equipment and the radio equipment controller. The CPRI (Common Public Radio Interface) specification is one example of such an interface definition. Depending on the used communications standard, the interface between the radio equipment and the radio equipment controller supports different data formats. For example, the functional split between the radio equipment and the radio equipment controller may be done in such a way that a generic interface based on In-Phase and Quadrature (IQ) data can be defined. For the UMTS radio access network, the REC provides access to the Radio Network Controller via the lub interface, whereas the RE serves as the air interface, called the Uu interface, to the user equipment. For WiMAX, the REC provides access to network entities (e.g. other BS (base stations), ASN-GW (Access Service Network Gateways)), whereas the radio equipment serves as the air interface to the subscriber station/mobile subscriber station (SS/MSS). A more detailed description of the functional split between both parts of a radio base station system is provided in the CPRI Specification V3.0. The CPRI specification should be considered as an example, only. The document WO 2006/040653 A1 describes further aspects of the communication between a radio equipment control node and multiple remote radio equipment nodes. The teachings disclosed herein could be combined or integrated with other specifications that describe master/slave-type links or networks.
A bidirectional interface between two directly connected ports, either between REC and RE, or between two REs, is called a “link”. In the CPRI specification, a link uses one transmission line per direction. A working link consists of a master port, a bidirectional cable, and a slave port. Therefore, the link is asymmetrical. The distinction between master port and slave port is important for synchronization, control and management (C&M) channel negotiation during start-up sequence, reset indication, and the start-up sequence. A port in master role is in charge to initiate and control the link setup procedure to the attached slave port. A port in slave role has to reply to incoming master setup attempts, but shall not initiate any connection setup.
The CPRI specification mentions some further conditions. For example, the ports of the REC are always master ports. At least one port of an RE should be a slave port. Optionally, an RE may have additional slave or master ports. The CPRI specification does not consider master/master and slave/slave links.
As to the direction of data flow, the term “downlink” describes the direction from REC to RE, or from master to slave for a link. The term “uplink” describes the opposite directions.
During the initialization phase of the internal network of a radio base station, the mentioned constraints need to be observed. At the same time every radio equipment (RE) needs to be connected to at least one radio equipment controller (REC) in order to be functional. A known approach for the network initialization procedure is to proceed step by step, starting at the REC. The master/slave roles of the ports of the radio equipments in the network are set centrally by the REC using dedicated control commands. The REC holds a preconfigured network topology map which contains the role settings for each individual node in the network.
A precondition for this network initialization procedure is that the REC has a database which contains a logical map of the network attached to it. This map must not have any deviations to the actual physical topology of the network.
In detail, the following actions are performed during the known network initialization procedure according to the CPRI specification:
All REs start up with all CPRI ports in slave mode.
The REC sets one port to master mode and starts establishing a connection to the first RE in the network. Any redundant ports remain silent.
Higher layer protocols for operations and maintenance (O&M) data are established between the REC and the RE.
The REC assigns master role to RE ports according to preconfigured REC internal map via O&M commands.
The newly assigned RE master ports establish links to slave ports of succeeding REs. In a similar manner as described above, higher layer protocols for O&M data are established between the first RE and the subsequent RE and the REC configures the subsequent RE according to the preconfigured REC internal map via O&M commands.
A complete network initialisation requires a certain amount of time, especially for large networks. One reason is that radio equipments that are far away (in terms of network topology) from the radio equipment controller depend on an accomplished configuration of the intermediary radio equipments. Another reason is that higher layer protocols are needed for the configuration of the radio equipments.
When a link loss occurs, the affected RE sets all ports to slave role, which in turn causes link loss and same action for all subsequent REs. The REC has to detect this situation and start to reconfigure the affected part of the network, starting with the last reachable node in the topology.
A mismatch between the topology map on the REC and the actual topology compromises the whole CPRI network, since no traffic (control data as well as payload data) can be routed beyond the point of mismatch.